Untreated metal surfaces are subject to corrosion which can lead to rust development, weakening, discoloration and failure of the surface. Thus metal substrates are typically treated by a variety of methods to make the surface less reactive and more corrosion resistant. In addition, metal surfaces are often subsequently coated with decorative or additional protective coatings such as resin coatings, primers, paints and other surface treatments.
One common mechanism of corrosion occurs galvanically when atmospheric oxygen oxidizes the metal of a metal substrate. The transfer of electrons from the metal to the oxygen at electrocatalytic sites leads to formation of various metal oxide corrosion products. A variety of treatments can be used to prevent corrosion of metal surfaces such as cold rolled steel, hot rolled steel, zinc, aluminum, and alloys thereof zinc- and zinc alloy-coated steel, as well as aluminum- and aluminum alloy-coated steel. These treatments include temporary oil-based rust preventatives, phosphate conversion coatings, inorganic and organic passivates, paints, and combinations thereof.
Temporary oil-based rust preventatives are used to provide short-term corrosion protection with ease of removal. They are not desirable for medium- or long-term corrosion protection alone, and when combined with other coatings make the surfaces unpaintable and by the nature of their handling are not suitable for a final consumer product.
Phosphate conversion coatings provide better corrosion protection and paintability, but require quite acidic conditions to operate, which is not desirable due to difficulty in handling the application chemicals, more troublesome waste treatment due to sludge generation, and extra wear on equipment. Such conversion coatings typically require subsequent treatments for maximum benefit.
Inorganic and organic passivates, such as those commonly applied to metal coil stock, provide high degrees of corrosion protection but possess several drawbacks. Many such products either contain chromium, are highly acidic, or both. Chrome is not desirable due to negative environmental considerations, toxicity, and more troublesome waste treatment. Highly acidic treatments are also prone to degrade processing equipment and pose concerns with worker exposure.
Another corrosion resistant coating approach has included the use of electrically conductive polymers, most commonly polyanilines (PANI), that conduct electrical currents without the addition of conductive inorganic substances. The conjugated double bonds in the polymers conduct electrons throughout the coating. Often, these electrically conductive polymer coatings may be doped with anions or they contain anions that release in response to a change in the electrical potential of the metal substrate. It has been suggested that the electrically conductive polymer acts as a cathode to supply sufficient current to form the protective barrier at the site of corrosion and simultaneously release anions which function as active inhibitors toward corrosion.
Electrically conductive polymers have found limited use within industrial coatings stemming from a number of drawbacks. In addition to having a high cost, the film formation properties of conductive polymers are not ideal and these polymers have limited solubility in organic solvents making application difficult. Consequently it is desirable to provide a mechanism for corrosion inhibition not reliant on use of conductive polymers. It is also desirable to provide a coating which responds to corrosion such that the means for inhibiting farther corrosive action is not depleted over time in the absence of significant corrosion. Coatings based solely on chrome have a number of undesirable attributes thus, it is desirable to provide coatings which are free of chrome which provide similar benefits against corrosion as chrome-based products. In other instances where chrome will continue to be used, it would be desirable to prolong the corrosion protection afforded by a chrome-containing coating composition. It would also be desirable to provide thin organic passivate compositions which are not acidic.